As a positive photoresist composition, a composition containing an alkali-soluble resin binder such as novolak resin, etc., and a naphthoquinonediazide compound as a photosensitive material is generally used.
Since the novolak resin as the binder is soluble in an aqueous alkali solution without being swelled therewith and also when the images formed are used as a mask for etching, the resin gives a high durability to the images against plasma etching, the novolak resin is particularly useful for the use of the present invention.
Also, the naphthoquinonediazide compound being used as the photosensitive material is a unique material in the point that the compound itself functions as a dissolution inhibitor for lowering the alkali solubility of a novolak resin but when the compound is decomposed by irradiated with light, an alkali-soluble material is formed to rather increase the alkali solubility of the novolak resin, and owing to the large change of the property by the action of light, the naphthoquinonediazide compound is particularly useful as the photosensitive material for a positive photoresist.
Hitherto, from such view points, many positive photoresists each containing the novolak resin and the naphthoquinonediazide series resin have been developed and practically used. In particular, the progress of the resist materials toward a high resolving power is remarkable and sufficient results have obtained in line width working up to submicrons.
Hitherto, for increasing the resolving power and obtaining an image reproduction of good pattern form, a resist having a high contrast (.gamma. value) has been considered to be useful and the technical development of resist compositions meeting such a purpose has been made. There are many publications disclosing such techniques. In particular, in regard to the novolak resin which is the main portion of positive photoresist, there are many patent applications on the monomer components, the molecule weight distributions, the synthesis methods, etc., and good results have been obtained. Also, in regard to the photosensitive material which is another main component in the present invention, compounds of many structures which are effective in obtaining a high contrast are disclosed. When a positive photoresist is prepared by utilizing these techniques, it has been able to develop a superhigh resolving power resist which can resolve a pattern of the dimension in the same degree as that of the wavelength of light.
However, an integrated circuit has more and more increased the integrated degree and in the production of a semiconductor substrate such as a very large scale integrated circuit (VLSI), etc., it has become necessary to work a ultra fine pattern composed of a line width such as 0.5 .mu.m or thinner. In such a use, a photoresist which stably obtains particularly high resolving power and has a wide development latitude for always ensuring a constant worked line width has been required. Also, it has been required that a resist residue does not form on the pattern of the resist after development for preventing the working defect of the circuit formed.
Also, in the formation of, in particular, a ultra fine pattern of 0.5 .mu.m or thinner, it has been found that, for example, even when a definite resolving power is obtained by a certain coated layer thickness, there is a phenomenon of deteriorating the resolving power obtained by slightly changing only the coated layer thickness (hereinafter, the phenomenon is referred to as "layer thickness reliance"). Astonishingly, it has also been found that even when the layer thickness slightly changes only several .mu.m-hundredths, the resolving power is largely changed and any typical positive photoresists commercially available at present have such a tendency more or less. Practically, when the thickness of the resist layer before light exposure changes in the range of .lambda./4n (wherein .lambda. is an exposure wavelength and n is the refractive index of the resist layer in the wavelength) to a definite layer thickness, the resolving power obtained thereby fluctuates.
As to the problem of the layer thickness reliance, the existence is indicated, e.g., by SPIE Proceedings, Vol. 1925, 626(1993) and it is described therein that the existence of the layer thickness reliance is caused by the multilayer reflection effect of light in the resist layer.
In the case of practically working a semiconductor substrate, a pattern is formed using a resist layer coated with a finely different layer thickness per positions by the unevenness of the surface of the substrate and the unevenness of the coated layer thickness. Accordingly, in the case of practicing fine working near the limit of the resolution using a positive photoresist, the layer thickness reliance has been one of hindrances.
Hitherto, for increasing the resolving power, many 1,2,-naphthoquinonediazide compounds of polyhydroxy compounds having specific structures are proposed. For example, these compound are disclosed in JP-A-57-63526, JP-A-60-163043, JP-A-62-10645, JP-A-62-10646, JP-A-62-150245, JP-A-63-220139, JP-A-64-76047, JP-A-1-189644, JP-A-2-285351, JP-A-2-296248, JP-A-2-296249, JP-A-3-48249, JPA-3-48250, JP-A-3-158856, JP-A-3-228057, and JP-A-4-365046 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), Patent Kohyo (PCT Japanese patent unexamined publication) No. 4-502519, U.S. Pat. Nos. 4,957,846, 4,992,356, 5,151,340, and 5,178,986, European Patent 530,148, etc.
However, even by using these photosensitive materials, the positive photoresists are yet insufficient in the view point of the reduction of the layer thickness reliance.
On the other hand, it is described, e.g., in JP-B-37-18015 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-58-150948, JP-A-2-19846, JP-A-2-103543, JP-A-3-228057, and JP-A-5-323597, European Patent 573,056, U.S. Pat. Nos. 3,184,310, 3,188,210, 3,130,047, 3,130,048, 3,130,049, 3,102,809, 3,061,430, and 3,180,733, West German Patent 938,233, SPIE Proceedings, Vol. 631, page 210, ibid., Vol. 1672, 231(1992), ibid., Vol. 1672, 262(1992), and ibid., Vol. 1925, 227(1993) that by utilizing a photosensitive material having a hydroxy group in the molecule, a resist having a high contrast and a high resolving power is obtained. Surely the photosensitive materials described in these publications are effective in elevating the contrast of resists containing them, but these are still insufficient in reducing the layer thickness reliance.
As described above, the manner of designing the composition of a resist material for reducing the layer thickness reliance and obtaining a high resolving power regardless of the layer thickness has not hitherto been known.
Also, with the increase of the integration of a semiconductor device, the requirement of a positive photoresist for particles has been more and more increased. As the term "1/10 rule" is generally said in the field of semiconductor, the particles of the size of larger than 1/10 of the smallest line wide of a semiconductor device give a bad influence on the yield of the device (Ultraclean Technology, Vol. 3, No. 1, 79(1991), etc.).
For reducing the particles, a means of using a superfine filter having a pore size of from 0.1 .mu.m to 0.05 .mu.m, etc., has been made at the production of photoresists and the means is useful for reducing the particles in the production of photoresists.
However, even when, for example, particles are less at the production of photoresists, it frequently happens that particles are increased in photoresists with the passage of time. The increase of particles in the photoresists with the passage of time is almost caused by a 1,2-quinonediazide photosensitive material and various investigations have hitherto been made for preventing the increase of particles with the passage of time.
For example, a method of using a photosensitive material obtained by acylating or sulfonylating a part of hydroxy groups of a polyhydroxy compound as disclosed in JP-A-62-178562, a method of using a mixture of a 1,2-naphthoquinonediazido-4-sulfonic acid ester and a 1,2-naphthoquinonediazido-5-sulfonic acid ester as disclosed in JP-A-62-284354, a method of using a thermally denatured 1,2-naphthoquinonediazide photosensitive material as disclosed in JP-A-63-113451, a method of reducing the residual catalyst in a photosensitive material as disclosed in JP-A-63-236030, a method of synthesizing a photosensitive material in the presence of an anion-exchange resin as disclosed in JP-A-63-236031, a method of mixing a photosensitive material with a solvent having an excellent solubility for the photosensitive material as disclosed in JP-A-61-260239 and JP-A-1-293340, etc., have hitherto been tried.